In conventional Cable Television (CATV) transmission systems, the conventional technology trend is to transmit signals optically via 1550 nm. The selection of the 1550 nm wavelength window is due to a lower fiber attenuation loss than in other alternatives. Furthermore, the availability of optical amplifiers in the 1550 nm window enables configuring systems with longer lengths of optical communication waveguides. However, in the 1550 nm wavelength window, the dispersion associated with optical waveguides results in distorted signals. For example, the interaction of the laser chirp, a frequency increase or decrease over a period of time, and fiber dispersion produces huge second order distortion and associated noise in CATV transmission systems and results in the inability to provide a directly modulated laser over the past decade for optical communication systems. The current technology used for 1550 nm transmitters is external modulation (EM) using external modulators. Furthermore, external modulation using external modulators is expensive and complex.
Recent changes associated with CATV system requirements, for example, smaller number of analog channels and more Quadrature Amplitude Modulation (QAM) channels, and Distributed Feedback (DFB) laser improvement, for example, smaller laser chirp parameter and better laser linearity results in the capability to use directly modulated laser transmitters for transmission of CATV signals in the 1550 nm window with the aid of dispersion correction. Dispersion correction is needed to correct the distortion caused by fiber dispersion correction.
There are basically two types of dispersion correction technologies, one being optical dispersion correction and the other is electronic dispersion correction. Optical dispersion technologies are normally expensive and optically lossy. Some optical dispersion correction technologies may also incur signal degradations, such as filter band limitation and optical non-linearity. In contrast to optical dispersion correction, electronic dispersion technology does not suffer from the issues as described above. Furthermore, electronic dispersion correction is cheaper to implement than optical dispersion correction.
While conventional electronic dispersion correction technology has advantages over the optical one, which can include low cost, being optically lossless and tunable, it also has disadvantages, such as limitations associated with dispersion correction, with respect to limiting the number of analog channels capable of transmission and reduced transmission waveguide distance.